Autonomous MEMS devices require similarly miniaturized power sources. However, most battery concepts are quite bulky. Recently, flexible thin-film batteries are being developed ideally suited for a variety of applications where small power sources are needed. Such flexible thin-film batteries are known from publications such as: [“Flexible Thin-film Batteries: A Global Technology”, Industry and Market Analysis Innovative Research and Products (iRAP), Inc. Jun. 1, 2007].
There are many different application areas of thin-film batteries. One application area is the use of thin-film batteries in RFID tags, see also publication: http://www.rfidjournal.com/article/view/94]. An RFID tag may be provided with an external energy source like a thin-film battery, which is then also being referred to as active RFID tags. Active RFID tags have a longer read range and offer better performance than passive RFID tags that draw their energy from a reader. The battery may be recharged by inductive coupling, or any other type of energy scavenger. In addition, a battery on board of the RFID tags can also enable additional embedded functionalities, such as sensor functionality.
Thin-film batteries could also be used to power smart cards or increase the range of identity badges with onboard biometrics. Another intriguing potential is the use of thin-film batteries in low-cost sensors.
Typically, thin-film batteries are suitable for applications requiring low-voltage power (1.5V-3.0V). Through stacking of multiple cells, higher voltages can be obtained by arranging the cells in series. By arranging cells in parallel, a larger capacity can be obtained. A battery's capacity depends primarily on the volume of both anode and cathode. Stacked thin film batteries are typically made by thin-film deposition, i.e. horizontal stacking of the different layers. In this configuration the capacity of the battery cells is mainly limited by the total area of the thin films.
U.S. Pat. No. 6,495,283 discloses a known thin-film battery. It discloses a battery having a trench structure which can increase an effective area per unit area. The battery forms trenches on thin film elements including a substrate, thereby increasing a contact interface between a cathode and an electrolyte and between the electrolyte and an anode, and simultaneously increasing an amount of an electrode per unit area. As a result, a high performance battery is provided, wherein the current density and the total current storage density are increased. Also, the charging speed after discharge is improved.
A problem with the known thin-film battery is that battery cell dimensions are not substantially determined by the design.